Image displays are widely used in a range of products, including commercial products such as televisions and computer systems, and industrial and military products such as sensor displays and data displays. The image display typically receives image information in electronic form and uses that image information to modulate a light-producing beam, a light source, or a light beam that is then viewed by the user of the display. The image information may be in either an analog or digital electronic form. Digital information is increasingly used because of its precision and because of the ability to generate and manipulate the electronic information with a computer prior to its use in modulating the light of the display.
A wide variety of image displays are available. In the familiar cathode ray tube, the image information modulates an electron beam incident upon a phosphor screen, which produces visible light responsive to the modulated electron beam. The cathode ray tube has long been used in conventional televisions. In the liquid crystal display, electronic image information alters the state of the pixels of a liquid crystal panel, which in turn modulates a light beam directed through the liquid crystal panel. In a projection display, a light beam is modulated with a digital micromirror device, a liquid crystal display, or otherwise, and is projected onto a display screen.
In some applications there is a need for a medium-sized display that has a small depth for its screen size, has high image contrast for viewing in a range of circumstances, is rugged, is all-digital, is stable in use over a range of temperatures, and is relatively inexpensive. In the development leading to the present invention, the inventors determined that the cathode ray tube is not acceptable because it requires a glass vacuum-tube environment, which is not sufficiently rugged because it can explode if the glass if punctured, and additionally is not compact. Available liquid crystal displays have contrast limitations and cannot display a full grey scale. Additionally, they have too great a temperature sensitivity of their performance, and sometimes must be placed in temperature-controlled enclosures that negate some of the compactness advantages they might otherwise achieve.
The projection display using a digital micromirror device provides a potential approach. However, projection displays in intermediate screen sizes and small depths are not available. In one application, the present inventors seek to provide a projection display that meets all of the requirements indicated above, has a 13 inch diagonal display size DD, and has a housing envelope depth HD that is less than 5 inches. The relation of large screen size DD and small housing envelope depth HD can be met in larger screen sizes such as used in home entertainment systems, but it cannot be met with existing configurations in intermediate screen sizes because may components do not scale down in size proportionate to the screen size. There is therefore a need for an improved projection display suitable for use in compact applications with an intermediate screen size, and is suitable for use in multi-display systems. The present invention fulfills this need, and further provides related advantages.